It is known that, in order to construct a descent and/or approach profile for an aircraft, in particular a transport airplane, a flight management system (FMS) of the aircraft defines an optimized vertical profile by performing a calculation in an upstream direction, that is to say a backward calculation. This backward calculation is carried out on the basis of the threshold of the runway or, in accordance with the type of approach, from a conventional point (such as a “missed approach point” or a “final end point”) up to the final cruise flight level (identified by a point TD (“top of descent”)), taking into account speed and/or altitude constraints inserted into the flight plan. A deceleration point DECEL is likewise calculated by the FMS system. The point DECEL corresponds to the beginning of the deceleration to the approach speed (VAPP). This point DECEL determines the limit between the descent and approach phases.
With this method of backward calculation, the first step is the calculation of the approach profile defined by:                a final approach profile calculated from the threshold of the runway up to a point FAF (“final approach fix”) or FAP (“final approach point”). This final approach profile is determined in the conventional way by a fixed slope angle, corresponding to the final part defined in the procedure; and        an intermediate approach profile from the point FAF/FAP to the deceleration point DECEL. Along this intermediate profile, the aircraft begins the deceleration from the point DECEL until the final approach speed (VAPP) generally reached at a height of 1000 feet above the ground.        
In order to calculate the approach profile, the FMS system considers that the deceleration point DECEL is reached in a clean configuration at the maximum speed (generally at 250 knots) or at a lower speed if constraints exist before the point DECEL. Then:                aerodynamic configuration change sequences are implemented with in particular a deployment of the slats and flaps,        the landing configuration is applied and the speed VAPP is reached at 1000 feet above the ground.        
In addition, the FMS system conventionally associates a type with each segment defined in the vertical profile. In accordance with the performance levels of the aircraft and the state of the aircraft and external conditions (mass, centre of gravity, altitude, speed, wind and temperature conditions, etc.), and the slope of the segment in question, the aircraft exhibits a specific deceleration capacity along a geometric segment. The deceleration capacity defines the type of geometric segment:                if the slope of the segment enables a deceleration which is sufficient for the segment to be able to be flown in a clean configuration (that is to say, without the slats and flaps deployed), said segment is said to be in the “clean airbrake” configuration;        if the slope of the segment does not allow a deceleration which is sufficient for the segment to be able to be flown in a clean airbrake configuration, but on the other hand allows it to be flown with the airbrakes half deployed (“half airbrake”), said segment is said to be in the “half airbrake configuration”; and        if the slope of the segment does not allow a deceleration which is sufficient for the segment to be able to be flown in the clean airbrake configuration, even with the airbrakes half deployed, said segment is said to be “too steep”.        
The type of the segment is evaluated at each altitude constraint, and at the changes of aerodynamic configurations, in particular at the transition from the clean airbrake configuration to the configuration 1 and at the transition from the configuration 1 to the configuration 2 (for a given slope, a segment may be in the clean airbrake configuration in configuration 2 and in the half airbrake configuration in configuration 1).
A segment which is too steep brings about vertical discontinuity. In this instance, the FMS system indicates that, taking into account the performance of the aircraft, the segment cannot be flown, even with the airbrakes half deployed, and it allows the pilot the choice of carrying out the appropriate action to overcome this problem (further deploying the airbrakes, anticipating the change of configuration or the deployment of the landing gear). However, from an operational point of view, a vertical discontinuity would have to be avoided to the greatest possible extent, primarily in the approach phase.
In order to overcome this problem, when a segment which is too steep is generated owing to the position of the configuration change point during the calculation of the backward approach profile, the FMS system conventionally makes provision for the current configuration and the current speed to be maintained (in a backward direction) as far as the end of the segment. Consequently:                a constant speed segment is created;        a segment which is too steep is prevented; and        the point DECEL is positioned higher and further from the destination, compared with the profile without anticipated (or “different”) modification of configuration in the backward calculation.        
An anticipated change of aerodynamic configuration creates better capacities for deceleration along the segment. This change anticipates the appearance of a segment which is too steep, but increases the altitude of the configuration change and thus the altitude of the point DECEL.
With the above logic used by the FMS system to calculate the approach profile, when the configuration change speed (VCC) from the current configuration to the following configuration is reached, the configuration change point is positioned in the profile and the type of the segment is defined. If, in accordance with the position of the configuration change point and altitude constraint(s), a segment which is too steep is provided, an anticipated configuration change logic is applied. The following (or downstream) configuration is maintained (in the backward calculation) and a constant speed is maintained.
A constant speed segment of this type (which is potentially very long) is the major cause of the positioning of the point DECEL at a high altitude.
This is because the position of the point DECEL (that is to say, the point of the beginning of deceleration to the approach speed) must be able to comply with operational considerations. The approach phase must begin at an altitude at which the aircraft is supposed to begin the deceleration to the approach speed. A point DECEL which is too high is not adapted either to the manner in which the pilots are accustomed to carrying out the descent and the approach, or to the speeds anticipated by air traffic control at such an altitude or distance from the final destination.